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参数资料
| 型号： | FAN2108MPX |
| 厂商： | Fairchild Semiconductor |
| 文件页数： | 11/14页 |
| 文件大小： | 0K |
| 描述： | IC REG BUCK SYNC ADJ 8A 25MLP |
| 标准包装： | 1 |
| 系列： | TinyBuck™ |
| 类型： | 降压（降压） |
| 输出类型： | 可调式 |
| 输出数： | 1 |
| 输出电压： | 0.8 V ~ 19 V |
| 输入电压： | 3 V ~ 24 V |
| PWM 型： | 电流模式 |
| 频率 - 开关： | 200kHz ~ 600kHz |
| 电流 - 输出： | 8A |
| 同步整流器： | 是 |
| 工作温度： | -10°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 25-MLP |
| 包装： | 标准包装 |
| 供应商设备封装： | 25-MLP（5x6） |
| 产品目录页面： | 1214 (CN2011-ZH PDF) |
| 其它名称： | FAN2108MPXDKR |
Calculating the Inductor Value
R ILIM = V RILIM / 10μA
(7)
Typically the inductor is set for a ripple current ( Δ I L ) of
10% to 35% of the maximum DC load. Regulators
requiring fast transient response use a value on the high
side of this range; while regulators that require very low
output ripple and/or use high-ESR capacitors restrict
The voltage V RILIM is made up of two components, V BOT
(which relates to the current through the low-side
MOSFET) and V RMPEAK (which relates to the peak
current through the inductor). Combining those two
voltage terms results in:
allowable ripple current.
R ILIM = (V BOT + V RMPEAK )/ 10μA
(8)
V OUT ? (1 - OUT )
L =
V
V IN
Δ I L ? f
(4)
R ILIM = {0.96 + (I LOAD * R DSON *K T *8)} +
{D*(V IN – 1.8)/(f SW *0.03*R RAMP )}/10μA
(9)
where f is the oscillator frequency.
Setting the Ramp Resistor Value
The internal ramp voltage excursion ( ? V RAMP ) during t ON
should be set to 0.6 V at nominal operating point. R RAMP
is approximately:
where:
V BOT = 0.96 + (I LOAD * R DSON *K T *8);
V RMPEAK = D*(V IN – 1.8)/(f SW *0.03*R RAMP );
I LOAD = the desired maximum load current;
R RAMP ( K Ω ) =
( V IN ? 1 . 8 ) ? V OUT
18 x 10 ? 6 ? V IN ? f
? 2
(5)
R DSON = the nominal R DSON of the low-side MOSFET;
K T = the normalized temperature coefficient for the
low-side MOSFET (on datasheet graph);
where frequency (f) is expressed in KHz.
Setting the Current Limit
The current limit system involves two comparators. The
MAX I LIMIT comparator is used with a V ILIM fixed-voltage
reference and represents the maximum current limit
allowable. This reference voltage is temperature
compensated to reflect the R DSON variation of the low-
side MOSFET. The ADJUST I LIMIT comparator is used
where the current limit needs to be set lower than the
V ILIM fixed reference. The 10 μA current source does not
track the R DSON changes over temperature, so change is
added into the equations for calculating the ADJUST
I LIMIT comparator reference voltage, as is shown below.
Figure 22 shows a simplified schematic of the over-
current system.
D = V OUT /V IN duty cycle;
f SW = Clock frequency in kHz; and
R RAMP = chosen ramp resistor value in k Ω .
After 16 consecutive, pulse-by-pulse, current-limit
cycles, the fault latch is set and the regulator shuts
down. Cycling V CC or EN restores operation after a
normal soft-start cycle (refer to the Auto-Restart
section) .
The over-current protection fault latch is active during
the soft-start cycle. Use 1% resistor for R ILIM .
Loop Compensation
The loop is compensated using a feedback network
around the error amplifier. Figure 23 shows a complete
RAMP
VERR
+
_
PWM
COMP
type-3 compensation network. For type-2 compensation,
eliminate R3 and C3.
PWM
MAX
V CC
VILIM
+
_
ILIMIT
10μA
ILIM
+
_
ADJUST
ILIMIT
ILIMTRIP
RILIM
Figure 22. Current-Limit System Schematic
Since the I LIM voltage is set by a 10 μA current source
into the R ILIM resistor, the basic equation for setting the
reference voltage is:
Figure 23. Compensation Network
Since the FAN2108 employs summing current-mode
architecture, type-2 compensation can be used for many
applications. For applications that require wide loop
bandwidth and/or use very low-ESR output capacitors,
V RILIM = 10μA*R ILIM
To calculate R ILIM :
? 2008 Fairchild Semiconductor Corporation
FAN2108 ? Rev. 1.0.2
(6)
11
type-3 compensation may be required.
Protection
www.fairchildsemi.com
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